mkimage: Correct file being closed twice in fit_import_data()

The code flows through to the end of the function, so we don't need another
close() before this. Remove it.

Reported-by: Coverity (CID: 138504)

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>